This invention relates generally to panels that are applied to framing in residential and other types of light construction. More particularly, the invention relates to panels that are able to resist lateral forces imposed by high wind and earthquake loads in regions where they are required by building codes. Such panels, commonly known as shear walls or diaphragms, must demonstrate shear resistance as shown in recognized tests, such as ASTM E72.
If one considers a simple box structure having panels fastened to framing, it can be seen that a strong lateral force acting against one side of the box (e.g., wind pressure) will tend to force the side walls resisting that force from a rectangular shape into a parallelogram. Not all sheathing panels are capable of resisting such forces, nor are they very resilient, and some will fail, particularly at points where the panel is fastened to the framing. Where it is necessary to demonstrate shear resistance, the sheathing panels are measured to determine the load which the panel can resist within the allowed deflection without failure.
The shear rating is generally based on testing of three identical 8xc3x978 ft (2.44xc3x972.44 m) assemblies, i.e., panels fastened to framing. One edge is fixed in place while a lateral force is applied to a free end of the assembly until the load is no longer carried and the assembly fails. The measured shear strength will vary, depending upon the thickness of the panel and the size and spacing of the nails used in the assembly. For example, a typical assembly, e.g., a nominal xc2xd inch (12.7 mm) thick plywood fastened with 8d nails (see the nail description below) to nominal 2xc3x974 inch (50.8xc3x97101.6 mm) wood studs spaced 16 inches (406.4 mm) apart (on centers), the nails being spaced 6 inches (152.4 mm) apart on the perimeter and 12 inches (304.8 mm) apart within the perimeter, would be expected to show a shear strength of 720 lbs/ft (1072 kg/m) before failure occurs. (Note that the measured strength will vary as the nail size and spacing is changed, as the ASTM E72 test provides.) This ultimate strength will be reduced by a safety factor, e.g., a factor of three, to set the design shear strength for the panel.
Sheathing panels used where a shear rating must be met usually are plywood or oriented strand board (OSB), which consist of pieces of wood that are glued together. These panels can provide the needed shear strength, but each is combustible and neither is durable when exposed to water. A panel made of hydraulic cement will resist water, but is much heavier than the wood panels and has insufficient shear strength. It is believed that there is no panel currently available which can provide the necessary shear strength, while avoiding the deficiencies of plywood or OSB panels.
As the thickness of the board affects its physical and mechanical properties, e.g., weight, load carrying capacity, racking strength and the like, the desired properties vary according to the thickness of the board. Thus, the desired properties which a shear rated panel with a nominal thickness of 0.5 inches (12.7 mm) should meet include the following.
The panel when tested according to ASTM 661 and American Plywood Association (APA) Test Method S-1 over a span of 16 inches (406.4 mm) on centers, should have an ultimate load capacity greater than 550 lbs (250 kg) under static loading, an ultimate load capacity greater than 400 lbs (182 kg) under impact loading and a deflection of less than 0.078 inches (1.98 mm) under both static and impact loading with a 200 lb (90.9 kg) load.
The racking shear strength of a 0.5 inch (12.7 mm) thick panel measured by the ASTM E72 test using the nail size and spacing described above should be at least 720 lbs/ft (1072 kg/m).
A 4xc3x978 ft, xc2xd inch thick panel (1.22xc3x972.4 m, 12.7 mm thick) should weigh no more than 99 lbs (44.9 kg) and preferably no more than 85 lbs (38.6 kg).
The panel should be capable of being cut with the circular saws used to cut wood.
The panel should be capable of being fastened to framing with nails or screws.
The panel should be machinable so that tongue and groove edges can be produced in the panel.
The panel should be dimensionally stable when exposed to water, i.e., it should expand as little as possible, preferably less than 0.1% as measured by ASTM C 1185.
The panel should not be biodegradable or subject to attack by insects or rot.
The panel should provide a bondable substrate for exterior finish systems.
The panel should be non-combustible as determined by ASTM E136.
After curing for 28 days, the flexural strength of a 0.5 inch (12.7 mm) thick panel having a dry density of no more than 65 lb/ft3 (1041 kg/m3) after being soaked in water for 48 hours should be at least 1700 psi (11.7 MPa), preferably at least 2500 psi (17.2 MPa), as measured by ASTM C 947. The panel should retain at least 75% of its dry strength.
It should be evident that plywood and OSB panels meet some, but not all, of the above performance characteristics. Thus, there is a need for improved panels which can meet the shear rating required in certain locations and which exceed the capability of the currently-used wood-based panels by providing non-combustibility and water durability.
Prior art hydraulic cement-based panels and structures also have not possessed the combination of low density, nailability and cuttability required to enable the panel to be cut or fastened (either nailed or screwed) with conventional carpentry tools.
The panels of the invention may generally be described as gypsum-cement compositions reinforced with glass fibers and, with the addition of microspheres, having reduced weight compared with hydraulic cement panels. The panels will satisfy performance requirements listed above and may be distinguished from other compositions to be discussed below which contain similar components, but are not capable of meeting the desired performance.
Gypsum-cement compositions are disclosed generally in U.S. Pat. Nos. 685,903; 5,858,083 and 5,958,131. In each patent, pozzolans are added, silica fume in the ""903 and ""803 patents and metakaolin in the ""131 patent. Aggregates and fiber additions are suggested, but panels meeting the requirements of the invention are not described.
Although glass fibers have been used to reinforce cement, they are known to lose strength with time since the glass is attacked by the lime present in cured cement. This may be offset, to some extent, by coating the glass fibers or by using a special alkali-resistant glass. Other fibers have been suggested to reinforce cement, such as metal fibers, wood or other cellulose fibers, carbon fibers, or polymer fibers.
Cement-based panels and structures have also contained lightweight particles of glass, ceramics and polymers in order to reduce weight, but at the expense of reduced strength. Other aggregates have been suggested, but they will not have the advantages of the lightweight particles.
In U.S. Pat. No. 4,379,729, three layers are used in panels intended to replace wood for concrete forms. The outer two layers are glass fiber reinforced cement, while the middle layer is cement containing hollow spheres. While such panels are subject to static loading, they are not required to meet the building code requirements where wind and earthquake loads are expected.
In Russian Patent No. SU 1815462, three layers are also used in making pipe, rather than panels. Again, the outer layers are made of glass fiber reinforced cement, while the middle layer contains both glass fiber and glass spheres.
A thick modular wall section, rather than a shear rated panel, is discussed in U.S. Pat. No. 4,259,824. Various aggregates, including glass fibers, are suggested to be useful.
In U.S. Pat. No. 5,154,874, a gypsum board including paper fibers is disclosed.
A gypsum-cement panel is discussed in Canadian Patent No. CA 2,192,724. The panel contains 10 to 35 wt. % of wood or paper fibers, rather than glass fibers. Similarly, in U.S. Pat. No. 5,371,989, a gypsum board is disclosed which has glass fiber mats on the exterior surfaces.
In International Publication No. WO 93/10972, an interior panel is described which includes low density aggregates surrounded with cement and disposed within a foamed cement continuous phase. The panels may include glass fibers.
Cellulose or glass fibers are suggested to replace asbestos fibers in cement panels in U.S. Pat. No. 4,808,229.
A layered panel is disclosed in Japanese Patent No. JP 62-238734A. Microspheres are used inside the panel, while cement reinforced with carbon or plastic fibers is used on the outer surfaces.
In U.S. Pat. No. 4,504,320, a glass-reinforced Portland cement is described that includes fly ash cenospheres and silica fume.
It will be evident from the above discussion that fiber reinforcement of cement has been used and that microspheres of glass, ceramic and polymer have been included to reduce weight. Other examples are found in Japanese Patent Nos. JP-2641707 B2, JP 53-034819, JP 54-013535 and JP 94-096473 B2, Swedish Patent No. SE 8603488, and U.K. Patent No. GB 1493203.
Despite all the effort which has gone into the reinforcement of cement, as indicated by the various patents and patent applications mentioned above, the present inventors believe that none of the panels currently available are able to replace plywood or OSB panels in applications where they must meet code required resistance to shear loads or have similar handling characteristics, e.g., cutting and nailing. In the following discussion, it will be shown that a gypsum-cement panel can be made which is capable of satisfying or exceeding the shear loadings now only possible with plywood or OSB panels.
The present invention achieves the combination of low density and ductility required for panel handling and nailability in one of the following three ways:
The use of lightweight ceramic microspheres uniformly distributed throughout the full thickness of the panel.
The use of a blend of lightweight ceramic and polymer microspheres throughout the full thickness of the panel, alternatively adjusting the amount of water used in forming the panel to provide an effect similar to that of polymer microspheres, or a combination thereof.
Creating a multi-layer panel structure containing at least one outer layer having improved nailability and cuttability. This is provided by using a higher water-to-reactive powder (defined below) ratio in making the outer layer(s) relative to the core of the panel or by incorporating lightweight polymer microspheres in substantial quantities in the outer layer(s) relative to the core of the panel, while the inner core corresponds to the previously described panels.
The first embodiment of the present invention is a lightweight, dimensionally stable panel reinforced with alkali-resistant glass fibers and containing ceramic microspheres. In the panel, the glass fibers and ceramic microspheres are uniformly distributed throughout a continuous phase comprising a cured aqueous mixture of reactive powders, i.e., calcium sulfate alpha hemihydrate, hydraulic cement, lime and an active pozzolan. The second embodiment of the present invention is a lightweight, dimensionally stable panel reinforced with alkali-resistant glass fibers and containing microspheres, which may be a blend of ceramic and polymer microspheres uniformly distributed throughout the continuous phase for the full thickness of the panel. Alternatively, the water-to-reactive powder ratio may be increased to achieve an effect similar to adding polymer microspheres, which may be replaced in whole or in part. The third embodiment of the present invention is a lightweight, dimensionally stable panel reinforced with alkali-resistant glass fibers using a multi-layer structure in which a core has one or two outer facing layers. In this embodiment, the outer layer (or layers) incorporates lightweight polymer microspheres in substantial quantities in a second continuous phase reinforced with glass fibers, the outer layer (or layers) disposed on a core having either ceramic microspheres or a blend of both ceramic and polymer microspheres uniformly distributed throughout a continuous phase, such blend optionally being determined by the water-reactive powder ratio and reinforced with alkali-resistant glass fibers. Alternatively, the outer layer (or layers) may be made with a higher water-to-reactive powder ratio than is used in the core of the panel to achieve an effect similar to adding polymer microspheres, which may be replaced in whole or in part.
In all three embodiments, when the panel is fastened to framing, as provided in the ASTM E72 test, it is capable of meeting or exceeding the shear loading required by building codes where the panels must be able to resist high wind or earthquake forces. The panels may also be used as structural subflooring or as flooring underlayment. In such applications, the panels preferably will employ a tapered tongue and groove joint.
In producing the panel of the first embodiment of the invention, ceramic microspheres are utilized as lightweight fillers. These microspheres are uniformly distributed throughout the full thickness of the panel. In the composition, the dry ingredients are the reactive powders (20 to 55 wt. % hydraulic cement, 35 to 75 wt. % calcium sulfate alpha hemihydrate, 5 to 25 wt. % pozzolan, and 0.2 to 3.5 wt. % lime on a dry basis), ceramic microspheres and alkali-resistant glass fibers, and the wet ingredients are water and superplasticizer. The dry ingredients and the wet ingredients are combined to produce the panel of the first embodiment of the invention. Of the total weight of dry ingredients, the panel of the invention preferably is formed from about 49 to 56 wt. % reactive powders, 35 to 42 wt. % ceramic microspheres and 7 to 12 wt. % alkali-resistant glass fibers. In the broad range, the panel of the invention is formed from about 35 to 58 wt. % reactive powders, 34 to 49 wt. % ceramic microspheres, and 6 to 17 wt. % alkali-resistant glass fibers, based on the total dry ingredients. The amounts of water and superplasticizer added to the dry ingredients are sufficient to accomplish the desired slurry fluidity needed from the processing considerations for any particular manufacturing process. The typical addition rates for water range between 35 to 60% of the weight of reactive powders (water to reactive powders 0.35-0.6/1) and those for superplasticizer range between 1to 8% of the weight of reactive powders. The glass fibers are monofilaments having a diameter of about 5 to 25 microns (micrometers), typically about 10 to 15 microns (micrometers). The monofilaments are bundled in several ways. In one typical configuration 100 fiber strands are combined into rovings containing about 50 strands. Other arrangements are possible. The length of the glass fibers will preferably be about 1 to 2 inches (25 to 50 mm) and broadly about 0.25 to 3 inches (6.3 to 76 mm), and the fiber orientation will be random in the plane of the panel.
In producing the panel of the second embodiment of the invention a blend of ceramic microspheres and polymer microspheres is utilized as lightweight fillers. It has been discovered that incorporation of polymer microspheres in the panel helps to achieve the combination of low density and better nailability required to enable the panel to be cut or fastened (either nailed or screwed) with conventional carpentry tools. Since the water-to-reactive powder ratio also affects density and nailability, it may be adjusted to provide a similar effect to that of the polymer microspheres, although polymer microspheres may be included and need not be completely replaced by adjusting the water-to-reactive powder ratio. It also has been found that the rheological properties of the slurry are improved substantially by utilizing a combination of ceramic and polymer microspheres in the composition. Therefore, in the second embodiment of the invention, the dry ingredients of the composition are the reactive powders described above (i.e., hydraulic cement, calcium sulfate alpha hemihydrate, pozzolan, and lime), ceramic microspheres, polymer microspheres, and alkali-resistant glass fibers, and the wet ingredients of the composition are water and superplasticizer. The dry ingredients and the wet ingredients are combined to produce the panel of the invention. The ceramic and polymer microspheres are uniformly distributed in the matrix throughout the full thickness of the panel. To achieve good fastening and cutting ability, the volume fraction of the polymer microspheres in the panel preferably is in the range of 7 to 15% of the total volume of dry ingredients. Of the total weight of dry ingredients, the panel of the invention preferably is formed from about 54 to 65 wt. % reactive powders, 25 to 35 wt. % ceramic microspheres, 0.5 to 0.8 wt. % polymer microspheres, and 6 to 10 wt. % alkali-resistant glass fibers. In the broad range, the panel of the invention is formed from about 42 to 68 wt. % reactive powders, 23 to 43 wt. % ceramic microspheres, up to 1.0 wt. % polymer microspheres, preferably 0.2 to 1.0 wt. %, and 5 to 15 wt. % alkali-resistant glass fibers, based on the total dry ingredients. The amounts of water and superplasticizer added to the dry ingredients are adjusted to accomplish the desired slurry fluidity needed from the processing considerations for any particular manufacturing process. If desired, additional water may be used instead of polymer microspheres to provide an effect on density and nailability similar to that of the polymer spheres, or both polymer spheres and additional water may be used. The typical addition rates for water range between 35 to 70% of the weight of reactive powders and those for superplasticizer range between 1 to 8% of the weight of reactive powders. If additional water is used, the ratio of water-to-reactive powders will be greater than 0.6/1 ( greater than 60% water based on the reactive powders) preferably  greater than 0.6/1 to 0.7/1, more preferably 0.65/1-0.7/1. When the ratio of water-to-reactive powders is adjusted to replace polymer spheres, the composition will be adjusted accordingly to produce aqueous mixtures having a consistency suitable for forming a panel of the invention.
The glass fibers are monofilaments having a diameter of about 5 to 25 microns (micrometers), typically about 10 to 15 microns (micrometers). As mentioned above, the monofilaments may be bundled in several ways, for example as 100 fiber strands, which may be combined into rovings containing about 50 strands. The length of the glass fibers preferably is about 1 to 2 inches (25 to 50 mm) and broadly about 0.25 to 3 inches (6.3 to 76 mm), and the fiber orientation will be random in the plane of the panel.
In the third embodiment of the invention, a multi-layer structure in the panel is created where a core has at least one outer layer thereon having improved nailability (fastening ability). This is achieved by incorporating substantial amounts of polymer microspheres in the outer layers, or by using a higher water-to-reactive powder ratio than is used in making the core or by a combination thereof. The core layer of the panel contains hollow ceramic microspheres uniformly distributed throughout the layer thickness or in some embodiments a blend of ceramic and polymer microspheres. As in the second embodiment, the water-to-reactive powder ratio in the core may be adjusted to provide a similar effect to that of the polymer microspheres.
However, the core should be made stronger than the outer layers and in general, the amount of polymer spheres used or the water-to-reactive powder ratio will be chosen so that the core of the panel has better nailability than one having only ceramic microspheres, but provides suitable shear strength. The dry ingredients of the core layer are the reactive powders discussed above (i.e., hydraulic cement, calcium sulfate alpha hemihydrate, pozzolan, and lime), microspheres (ceramic alone or a blend of ceramic and polymer microspheres) and alkali-resistant glass fibers, and the wet ingredients of the core layer are water and superplasticizer. The dry ingredients and the wet ingredients are combined to produce the core layer of the panel of the invention. Of the total weight of dry ingredients, the core layer of the panel of the invention preferably is formed from about 49 to 56 wt. % reactive powders, 35 to 42 wt. %. ceramic microspheres and 7 to 12 wt. % alkali-resistant glass fibers or alternatively, about 54 to 65 wt. % reactive powders, 25 to 35 wt. % ceramic microspheres, 0.5 to 0.8 wt. % polymer microspheres, and 6 to 10 wt. % alkali-resistant glass fibers. In the broad range, the outer layer(s) of a multi-layer panel or core layer of the panel of the invention is formed from about 35 to 58 wt. % reactive powders, 34 to 49 wt. % ceramic microspheres, and 6 to 17 wt. % alkali-resistant glass fibers based on the total dry ingredients or alternatively, about 42 to 68 wt. % reactive powders, 23 to 43 wt. % ceramic microspheres, up to 1.0 wt. % polymer microspheres, preferably 0.2 to 1.0 wt. %, and 5 to 15 wt. % alkali-resistant glass fibers. The amounts of water and superplasticizer added to the dry ingredients are adjusted to accomplish the desired slurry fluidity needed from the processing considerations for any particular manufacturing process. The typical addition rates for water range between 35 to 70% of the weight of reactive powders and those for superplasticizer range between 1 to 8% of the weight of reactive powders.
The dry ingredients of the outer layer(s) are the reactive powders (hydraulic cement, calcium sulfate alpha hemihydrate, pozzolan, and lime), ceramic microspheres, polymer microspheres and alkali-resistant glass fibers, and the wet ingredients of the outer layer(s) will be water and superplasticizer. The dry ingredients and the wet ingredients are combined to produce the outer layer(s) of the panel of the invention. In the outer layer(s) of the panel, where polymer microspheres are incorporated in substantial quantities to furnish good fastening and cutting ability to the panel, the volume fraction of the polymer microspheres in the outer layers of the panel preferably is in the range of 7 to 15% of the total volume of dry ingredients. Of the total weight of dry ingredients, the outer layers of the panel of the invention preferably are formed from about 54 to 65 wt. % reactive powders, 25 to 35 wt. % ceramic microspheres, 0.5 to 0.8 wt. % polymer microspheres, and 6 to 10 wt. % alkali-resistant glass fibers. In the broad range, the outer layer(s) will be formed from about 42 to 68 wt. % reactive powders, 23 to 43 wt. % ceramic microspheres, up to 1.0 wt. % polymer microspheres, and 5 to 15 wt. % alkali-resistant glass fibers, based on the total dry ingredients. The amounts of water and superplasticizer added to the dry ingredients will be adjusted to accomplish the desired slurry fluidity needed from the processing considerations for any particular manufacturing process. The typical addition rates for water will range between 35 to 70% of the weight of reactive powders (greater than 60% if intended to improve nailability) and those for superplasticizer will range between 1 to 8% of the weight of reactive powders. The preferred thickness of the outer layer(s) ranges between {fraction (1/32)} to {fraction (4/32)} inches (0.8 to 3.2 mm). If only one outer layer is used, it should be less than xe2x85x9c of the total thickness of the panel.
In both the core and outer layer(s), the glass fibers are monofilaments having a diameter of about 5 to 25 microns (micrometers), typically about 10 to 15 microns (micrometers). The monofilaments may be bundled in several ways, for example as 100 fiber strands, which may be combined into rovings containing about 50 strands. The fiber length will preferably be about 1 to 2 inches (25 to 50 mm) and broadly about 0.25 to 3 inches (6.3 to 76 mm), and the fiber orientation will be random in the plane of the panel.
In another aspect, the invention is a method for making the shear resistant panels just described. An aqueous slurry of the reactive powders (i.e., calcium sulfate alpha hemihydrate, hydraulic cement, active pozzolan and lime), and the microspheres (ceramic alone or a blend of ceramic and polymer microspheres) is prepared and then deposited in thin layers in a panel mold while combining the slurry with short chopped glass fibers, and producing a uniformly mixed core material. In the third embodiment, all layers (i.e., core, and one or two outer layers) of the panel are formed using the same procedure. The aqueous slurry for the core layer contains either only ceramic microspheres or a blend of ceramic and polymer microspheres, the slurry for the outer layers contains polymer microspheres in larger quantities than are used in the core, in order to provide outer layers which have good nailability, while providing suitable shear strength to the core layer.